Argonne scientists have used the Mira supercomputer to identify and to improve a new mechanism for eliminating friction, which fed into the development of a hybrid material that exhibited superlubricity—a state in which friction essentially disappears—at the macroscale—i.e., at engineering scale—for the first time. A paper on their work was published in the journal Science.

They showed that superlubricity can be realized when graphene is used in combination with nanodiamond particles and diamond-like carbon (DLC). Simulations showed that sliding of the graphene patches around the tiny nanodiamond particles led to nanoscrolls with reduced contact area that slide easily against the amorphous diamond-like carbon surface, achieving incommensurate contact and a substantially reduced coefficient of friction (~0.004).

It is estimated that nearly one third of the fuel used in automobiles is spent to overcome friction, while wear limits mechanical component life. Even a modest 20% reduction in friction can substantially affect cost economics in terms of energy savings and environmental benefits. In that context, superlubricity is desirable for various applications and therefore is an active area of research. To date, superlubricity has been primarily realized in a limited number of experiments involving atomically smooth and perfectly crystalline materials and supported by theoretical studies.

Superlubricity has been demonstrated for highly oriented pyrolytic graphite (HOPG) surfaces, as well as for multiwalled carbon nanotubes (MWCNTs), when the conditions for incommensurate contacts are met in a dry environment. Because these conditions are due to the incommensurability of lattice planes sliding against each other, they are referred to as structural lubricity and restricted to material interactions at the nanoscale. At the macroscale, this structural effect (hence, superlubricity) is lost because of the structural imperfections and disorder caused by many defects and deformations.

Low friction has recently been observed in centimeter-long double-walled carbon nanotubes with perfect atomic structures and long periodicity. Ultralow friction in disordered solid interfaces, such as self-mated DLC films and in fullerene-like nanoparticles such as molybdenum disulfide (MoS2), has been observed under specific environmental and sliding conditions. However, the exact superlubricity mechanism in the above cases is still debatable and is not realized for industrial applications. In recent studies at the nano- and macroscale, graphene has shown a potential to substantially lower friction and wear under specific conditions. However, sustained macroscale superlubricity, particularly at engineering scales, has yet to be demonstrated.

—Berman et al.

In this schematic of the superlubricity system, the gold represents nanodiamond particles; the blue is a graphene nanoscroll; green shows underlying graphene on silicon dioxide; and the black structures are the diamond-like carbon interface. Credit: Sanket Deshmukh, Joseph Insley, and Subramanian Sankaranarayanan, Argonne National Laboratory. Click to enlarge.

This animation depicts the mechanism in which graphene patches (blue) spontaneously roll around nanodiamonds (gold) to enable sustained superlubricity.

Prior to the computational work, Argonne scientists Ali Erdemir, Anirudha Sumant, and Diana Berman were studying the hybrid material in laboratory experiments at Argonne’s Tribology Laboratory and the Center for Nanoscale Materials, a DOE Office of Science User Facility. The experimental setup consisted of small patches of graphene (a two-dimensional single-sheet form of pure carbon) sliding against a DLC-coated steel ball.

The graphene-DLC combination was registering a very low friction coefficient (a ratio that measures the force of friction between two surfaces), but the friction levels were fluctuating up and down for no apparent reason. The experimentalists were also puzzled to find that humid environments were causing the friction coefficient to shoot up to levels that were nearly 100 times greater than measured in dry environments.

Using Mira, the ALCF’s 10-petaflops IBM Blue Gene/Q supercomputer, the researchers replicated the experimental conditions with large-scale molecular dynamics simulations aimed at understanding the underlying mechanisms of superlubricity at an atomistic level.

This led to their discovery of the graphene nanoscrolls. The material’s fluctuating friction levels were explained by the fact that the nanoscrolls themselves were not stable. The researchers observed a repeating pattern in which the hollow nanoscrolls would form, and then cave in and collapse under the pressure of the load.

The friction was dipping to very low values at the moment the scroll formation took place and then it would jump back up to higher values when the graphene patches were in an unscrolled state.

—Sanket Deshmukh, ALCF

The computational scientists had an idea to overcome this issue. They tried incorporating nanodiamond particles into their simulations to see if the hard material could help stabilize the nanoscrolls and make them more permanent. The simulations proved successful.

The graphene patches spontaneously rolled around the nanodiamonds, which held the scrolls in place and resulted in sustained superlubricity. The simulation results fed into a new set of experiments with nanodiamonds that confirmed the same.

The beauty of this particular discovery is that we were able to see sustained superlubricity at the macroscale for the first time, proving this mechanism can be used at engineering scales for real-world applications. This collaborative effort is a perfect example of how computation can help in the design and discovery of new materials.

—Subramanian Sankaranarayanan, Argonne computational nanoscientist, who led the simulation work at ALCF

Unfortunately, the addition of nanodiamonds did not address the material’s aversion to water. The simulations showed that water suppresses the formation of scrolls by increasing the adhesion of graphene to the surface.

While this greatly limits the hybrid material’s potential applications, its ability to maintain superlubricity in dry environments is a significant breakthrough.

The research team is in the process of seeking a patent for the hybrid material, which could potentially be used for applications in dry environments, such as computer hard drives, wind turbine gears, and mechanical rotating seals for microelectromechanical and nanoelectromechanical systems.

Adding to the material’s appeal is a relatively simple and cost-effective deposition method called drop casting. This technique involves spraying solutions of the materials on moving mechanical parts. When the solutions evaporate, it would leave the graphene and nanodiamonds on one side of a moving part, and diamond-like carbon on the other side.

Deshmukh expects the nanoscroll mechanism to spur future efforts to develop materials capable of superlubricity for a wide range of mechanical applications.

The Argonne team will continue its computational studies to look for ways to overcome the barrier presented by water.

We are exploring different surface functionalizations to see if we can incorporate something hydrophobic that would keep water out. As long as you can repel water, the graphene nanoscrolls could potentially work in humid environments as well.

—Subramanian Sankaranarayanan

Mira. The team’s groundbreaking nanoscroll discovery would not have been possible without a supercomputer like Mira. Replicating the experimental setup required simulating up to 1.2 million atoms for dry environments and up to 10 million atoms for humid environments.

The researchers used the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code to carry out the computationally demanding reactive molecular dynamics simulations. With the help of ALCF catalysts, a team of computational scientists who work directly with ALCF users, they were able to overcome a performance bottleneck with the code’s ReaxFF module, an add-on package that was needed to model the chemical reactions occurring in the system.

The ALCF catalysts, in collaboration with researchers from IBM, Lawrence Berkeley National Laboratory, and Sandia National Laboratories, optimized LAMMPS and its implementation of ReaxFF by adding OpenMP threading, replacing MPI point-to-point communication with MPI collectives in key algorithms, and leveraging MPI I/O. Altogether, these enhancements allowed the code to perform twice as fast as before.

With the recent announcement of Aurora, the ALCF’s next-generation supercomputer, Sankaranarayanan is excited about where this line of research could go in the future.

Given the advent of computing resources like Aurora and the wide gamut of the available two-dimensional materials and nanoparticle types, we envision the creation of a lubricant genome at some point in the future. Having a materials database like this would allow us to pick and choose lubricant materials for specific operational conditions.

—Subramanian Sankaranarayanan

The work was funded by the DOE Office of Science. Computing time at the ALCF was allocated through DOE’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program.

Contributors to the code optimization work include Nichols A. Romero, Wei Jiang, and Chris Knight from the ALCF; Paul Coffman from IBM; Hasan Metin Aktulga from Lawrence Berkeley National Laboratory (now at Michigan State University); and Tzu-Ray Shan from Sandia National Laboratories.